1. Field of the Invention
The present invention relates to a drive circuit for an electroluminescence (hereinafter referred to as EL) display apparatus comprising an electroluminescence device and thin-film transistors (hereinafter referred to as TFT).
2. Description of the Prior Art
In recent years, EL display apparatuses using EL devices have gained attention as display apparatuses to replace CRTs and LCDs.
Furthermore, display apparatuses using TFTs as the switching devices for driving the EL device are being researched and developed.
FIG. 1 shows a circuit diagram of an organic EL display apparatus of the prior art.
According to the same diagram, a display pixel 1 of the organic EL display apparatus of the prior art comprises a first TFT 100, a second TFT 200, a holding capacitor 300, and an organic EL device 400.
A gate signal line G, which supplies a gate signal, and a drain signal line D, which supplies a drain signal, cross, and in the vicinity of the intersection of both signal lines G and D there are provided the organic EL device 400 and the TFTs 100, 200 for driving the organic EL device 400.
First, the first TFT 100 comprises a gate electrode 110, which is connected to the gate signal line G and supplied with the gate signal, a drain electrode 120, which is connected to the drain signal line D and supplied with the drain signal, and a source electrode 130, which is connected to a gate electrode 210 of the second TFT 200 and to the holding capacitor 300.
Next, the second TFT 200 comprises the gate electrode 210, which is connected to the source electrode 130 of the first TFT 100, a source electrode 220, which is connected to an anode 410 of the organic EL device 400, and a drain electrode 230, which is connected to a driving power supply 500 for supplying power to the organic EL device 400 so as to drive the organic EL device 400.
Furthermore, the organic EL device 400 comprises the anode 410, which is connected to the source electrode 220 of the second TFT 200, a cathode 420, which is connected to a common power supply terminal 600, and a light emitting device layer 430, which is sandwiched between the anode 410 and the cathode 420.
When the gate signal from the gate signal line G is supplied to the gate electrode 110 of the first TFT 100, the first TFT 100 turns on and the drain signal that was supplied from the drain signal line D is applied to the gate electrode 210 of the second TFT 200 and to the holding capacitor 300. As a result, the second TFT 200 turns on and a current flows, corresponding to the gate voltage of the second TFT 200, from the driving power supply 500 to the organic EL device 400 so that the light emitting device layer 430 of the organic EL device 400 emits light.
The organic EL device 400 is deposited in a sequence of the anode 410 formed from a transparent electrode, such as indium tin oxide (ITO), a first hole transport layer formed from 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine (MTDATA), a second hole transport layer formed from N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (TPD), a light emitting layer formed from bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq2) including a Quinacridone derivative, the light emitting device layer 430 formed from various electron transport layers formed from Bebq2, and the cathode 420 formed from a magnesium-indium alloy.
The organic EL device 400 is deposited in a sequence of the anode 410 formed from a transparent electrode, such as indium tin oxide (ITO), a first hole transport layer formed from 4,4xe2x80x2-bis(3-methylphenylphenylamino)biphenyl (MTDATA), a second hole transport layer formed from 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylanine (TPD), a light emitting layer formed from 10-benzo[h]beryllium-benzoquinolinol complex (Bebq2) including a Quinacridone derivative, the light emitting device layer 430 formed from various electron transport layers formed from Bebq2, and the cathode 420 formed from a magnesium-indium alloy.
In the organic EL device, holes injected from the anode and electrons injected from the cathode are recombined within the light emitting layer so as to excite the organic molecules forming the light emitting layer and generate an exciton. In the process where the exciton deactivates, light is released from the light emitting layer. This release of light to the outside from the transparent anode through the transparent insulating substrate results in light being emitted.
On the other hand, it is necessary for the EL device in each display pixel to emit the same quantity of light so that a uniform and stable display is obtained at the surface of the EL display apparatus. However, since the characteristic of each second TFT 200 that is provided in each display pixel is not uniform, the currents supplied to the EL devices by the drive circuit for the EL display apparatus in the prior art cannot be kept uniform, thus resulting in a problem where the non-uniform currents appear as an uneven display among the display pixels.
Namely, the size of each second TFT varies, due to deviations in mask patterns during the manufacture of the TFTs and so forth, so that the current flowing to each drain varies even though the same gate voltage is applied to each second TFT. Therefore, the current supplied to the EL device differs with each display pixel and appears as an uneven display.
In view of the shortcomings of the above-mentioned prior art, it is therefore an object of the present invention to provide an EL display apparatus, in particular a drive circuit for the EL device, designed to improve the uniformity of light emission among display pixels and to easily enable the current supply to the EL device to be controlled.
The electroluminescence display apparatus of the present invention for performing display operations by an electroluminescence device, which comprises an anode and a cathode, emitting light, comprises: a first thin-film transistor, of which a source electrode is connected to a holding capacitor, a drain electrode is connected to a drain signal line, and a gate electrode is connected to a gate signal line; a second thin-film transistor, of which the drain electrode is connected to a driving power supply of the above-mentioned electroluminescence device, and the gate electrode is connected to the source electrode of the above-mentioned first thin-film transistor; a third transistor and a fourth transistor, which are connected between the source electrode of the above-mentioned second thin-film transistor and the anode of the above-mentioned electroluminescence device, for being switched in accordance with a predetermined external signal that is applied to respective gate electrodes; and a charging capacitor, which is connected between the above-mentioned third thin-film transistor and fourth thin-film transistor.
The electroluminescence display apparatus for performing display operations by causing the electroluminescence device, which comprises the anode and cathode, to emit light, comprises: the first thin-film transistor, of which the source electrode is connected to the holding capacitor, the drain electrode is connected to the drain signal line, and the gate electrode is connected to the gate signal line; the second thin-film transistor, of which the drain electrode is connected to the driving power supply of the above-mentioned electroluminescence device, and the gate electrode is connected to the source electrode of the above-mentioned first thin-film transistor; a first diode and a second diode, which are connected in series in a forward direction toward the anode of the above-mentioned luminescence device from the above-mentioned second thin-film transistor between the source electrode of the second thin-film transistor and the anode of the electroluminescence device; and a charging capacitor, which is connected between the above-mentioned first diode and the above-mentioned second diode. The driving power supply generates an output, the voltage of which changes periodically.
The electroluminescence display apparatus for performing display operations by causing the electroluminescence device, which comprises the anode and cathode, to emit light, comprises: a first switching device for receiving a display signal in accordance with a selection signal; the holding capacitor, which is connected to the first switching device, for holding the received display signal for a fixed period; a second switching device, which is connected between the holding capacitor and the first switching device, for operating by receiving at a control electrode the display signal voltage that was held by the holding capacitor and for outputting a current from the driving power supply of the above-mentioned electroluminescence device; a third switching device and a fourth switching device, which are disposed in this order between the second switching device and the anode of the electroluminescence device; and a charging capacitor, which is disposed between the third switching device and the fourth switching device, is charged from current that is output from the driving power supply via the second switching device and third switching device by operation of the third switching device. The third switching device is operated so that the charging capacitor is charged up to a voltage corresponding to a display signal voltage that was applied to the control electrode of the second switching device. The fourth switching device is operated so as to drive the electroluminescence device by applying an electric charge that was stored in the charging capacitor to the anode of the electroluminescence device.
The above-mentioned electroluminescence display apparatus has the above-mentioned first and second switching devices and the above-mentioned third and fourth switching devices respectively configured from thin-film transistors. Furthermore, the third and fourth switching devices operate alternately by receiving at respective gate electrodes, which are control electrodes, an external signal, which inverts at a fixed period shorter than a display signal holding period to the holding capacitor.
In another aspect of the present invention, the above-mentioned electroluminescence display apparatus has: the first and second switching devices configured from thin-film transistors, and the third and fourth switching devices configured from diodes connected in series in a forward direction from the second switching device to the anode of the electroluminescence device; the cathode of the electroluminescence device connected to the driving power supply of the electroluminescence device; the driving power supply of the electroluminescence device inverts an output level at a fixed period shorter than the display signal holding period of the holding capacitor; and an output from the driving power supply of the electroluminescence device alternately operate the third switching device and the fourth switching device so as to perform charging and discharging of the charging capacitor.
In the above-mentioned electroluminescence display apparatus: the electroluminescence device is a current-driven device causing an organic light emitting layer, which is sandwiched by the anode and the cathode and where holes are injected from the anode and electrons are injected from the cathode, to emit light.
If the electroluminescence device is driven according to the configurations described above, even with variations present in each second thin-film transistor or in the current characteristics of each second switching device, the charging capacitor can be charged from the driving power supply with a voltage substantially equal to the voltage that is applied to the gate electrode of the operating second thin-film transistor (second switching device). Therefore, the current that is supplied to the electroluminescence device corresponds to the voltage charged to the charging capacitor, namely, the voltage (display signal voltage held in the holding capacitor) that is applied to the gate electrode of the second thin-film transistor (second switching device). For this reason, variations in light emission of the electroluminescence device due to variations in characteristics of the second thin-film transistors are prevented so as to improve the display quality of the display apparatus.
In particular, since current without variations can be supplied to the current-driven organic EL device, it is possible to provide a high-quality organic EL display apparatus.